While singing her single "Whip My Hair," Bieber's manager Scooter Braun and "swagger coach" Ryan Good popped out of nowhere in wigs and makeup -- and were quickly followed by Justin himself, who couldn't resist adding some much needed cowbell to the tune.

Willow's brother, Jaden, also joined in on the action -- and started whipping his hair around his surprised sis on stage.

It doesn't look like Willow's going down without a fight though -- after the show she tweeted "my big brothers Justin and Jaden pranked me on stage tonight...#payback!!!